Add universum-prelude

Problem: in some projects, like `xrefcheck`, we would like to use
`universum` without constant explicit `import Universum`.

Solution: add a dedicated package that allows doing so.

One another way is using mixins feature, but it does not yet work really
well (`stack ghci` tends to fail, as well as Haskell language server),
so we better look at this feature again at its better times.

README.md

@@ -99,6 +99,43 @@ How to use Universum [↑](#structure-of-this-tutorial)
 Okay, enough philosophy. If you want to just start using `universum` and
 explore it with the help of compiler, set everything up according to the instructions below.
 
+There are three primary ways.
+
+### Implicit path
+
+Add a dependency on `universum-prelude` that provides `Prelude` with all the `universum` functionality.
+
+Also, replace `base` dependency with `base-noprelude` to avoid conflicts.
+
+### Modern implicit path
+
+Using [mixins](https://cabal.readthedocs.io/en/3.4/cabal-package.html#pkg-field-mixins) feature to provide `Universum` module as `Prelude`. In the configuration this may look like:
+
+```cabal
+mixins:
+  base hiding (Prelude),
+  universum (Universum as Prelude, Universum.Unsafe as Unsafe)
+```
+
+for `.cabal` file and
+
+```yaml
+dependencies:
+  - name: base
+    mixin: [hiding (Prelude)]
+    version: "< 4.15"
+  - name: universum
+    mixin: [(Universum as Prelude), (Universum.Unsafe as Unsafe)]
+```
+
+for `package.yaml`.
+
+Note that at the moment, the mixins feature is not widely supported by the Haskell ecosystem.
+
+### Explicit path
+
+This path for the case when you prefer using explicit `Universum` imports.
+
 Disable the built-in prelude at the top of your file:
 
 ```haskell

prelude/CHANGES.md

@@ -0,0 +1,5 @@
+0.1
+=====
+
+* Initial release.
+  Providing `Prelude` and `Unsafe` modules.

prelude/LICENSE

@@ -0,0 +1,20 @@
+The MIT License (MIT)
+Copyright (c) 2021 Serokell
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to
+deal in the Software without restriction, including without limitation the
+rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
+sell copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in
+all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
+IN THE SOFTWARE.

prelude/README.md

@@ -0,0 +1,9 @@
+Universum-prelude
+=================
+
+For medium-sized projects, it is tedious to write `import Universum` in every single module, while switching to a project-custom prelude may be not yet desired.
+
+This package re-exports `Universum` module as `Prelude`, effectively allowing implicit import of universum.
+When adding a dependncy on `universum-prelude` to your project, you will need to replace `base` dependency with `base-noprelude` to avoid conflicts.
+
+Additionally, `Universum.Unsafe` is re-exported as `Unsafe` module.

prelude/src/Prelude.hs

@@ -0,0 +1,5 @@
+module Prelude
+  ( module Universum
+  ) where
+
+import Universum

prelude/src/Unsafe.hs

@@ -0,0 +1,5 @@
+module Unsafe
+  ( module Universum.Unsafe
+  ) where
+
+import Universum.Unsafe

prelude/universum-prelude.cabal

@@ -0,0 +1,54 @@
+cabal-version:       2.2
+name:                universum-prelude
+version:             1.0
+synopsis:            Allows implicit Universum importing.
+description:         This package exports Prelude module that is just a re-export of Universum.
+                     Can be used with base-noprelude package to implicitly import Universum.
+                     Most suitable for medium-sized projects.
+homepage:            https://github.com/serokell/universum
+license:             MIT
+license-file:        LICENSE
+author:              @serokell
+maintainer:          Serokell <[email protected]>
+copyright:           2021 Serokell
+category:            Prelude
+stability:           stable
+build-type:          Simple
+bug-reports:         https://github.com/serokell/universum/issues
+extra-doc-files:     CHANGES.md
+                   , README.md
+
+source-repository head
+  type:     git
+  location: [email protected]:serokell/universum.git
+
+common common-options
+  build-depends:       base >= 4.8 && < 5
+  ghc-options:
+                       -- Source: https://medium.com/mercury-bank/enable-all-the-warnings-a0517bc081c3
+                       -Weverything
+                       -Wno-missing-exported-signatures
+                       -Wno-missing-import-lists
+                       -Wno-missed-specialisations
+                       -Wno-all-missed-specialisations
+                       -Wno-unsafe
+                       -Wno-safe
+                       -Wno-missing-local-signatures
+                       -Wno-monomorphism-restriction
+                       -Wno-implicit-prelude
+  if impl(ghc >= 8.10.1)
+    ghc-options:       -Wno-prepositive-qualified-module
+                       -Wno-inferred-safe-imports
+
+  default-language:    Haskell2010
+
+library
+  import:              common-options
+  hs-source-dirs:      src
+  exposed-modules:
+                       Prelude
+                       Unsafe
+
+  build-depends:       universum
+  ghc-options:         -Wimplicit-prelude
+  default-extensions:  NoImplicitPrelude

stack.yaml

@@ -1 +1,5 @@
 resolver: lts-15.11
+
+packages:
+- .
+- prelude